To avoid these difficulties, the use of a pressureless sintering technique for the manufacture of self-bonded lanthanum hexaboride shaped bodies was investigated by Russian authors at the end of the 1960's. A summary of the results obtained is given by H. Pastor in the book "Boron and Refractory Borides", Ed. V. I. Matkovich, Springer-Verlag Berlin-Heidelberg-New York 1977, pages 463 to 466. As can be seen from the book, densities of up to 82% of the theoretical density (noted as % TD herein) were achieved when using relatively coarse-grained lanthanum hexaboride powders (particle sizes in the range of from &lt;2.8 .mu.m to &gt;6.4 .mu.m) by sintering small sample in vacuo at 2400.degree. C. However considerable grain growth occurred (250 to 300 .mu.m) and there were substantial weight losses due to evaporation (up to 15% by weight). Embedding the samples to be sintered in lanthanum hexaboride powder made it possible to reduce the weight losses (&lt;5% by weight).
When very fine lanthanum hexaboride power (particle sizes 1 .mu.m) were used, densities close to the theoretical value were achieved by sintering under argon at from 2000.degree. to 2450.degree. C. or under hydrogen at from 1800.degree. to 2200.degree. C. However, the greatest grain growth occurred when sintering under hydrogen. For example, the following grain sizes were formed in the finished sintered body after 2 hours' sintering at 2200.degree. C.: 20 to 25 .mu.m in vacuo, 60 um under argon and 150 .mu.m under H.sub.2. Moreover, it was demonstrated that, when using commercial lanthanum hexaboride powders, high density is difficult to achieve in the sintering process because the impurities evaporate.
Self-bonded lanthanum hexaboride shaped bodies, manufactured by pressureless sintering, were consequently either not sufficiently dense or had an irregular density distribution. In addition the high weight losses occurring during the sintering process could be reduced but not eliminated by embedding in lanthanum hexaboride powder. By using a mixture of lanthanum hexaboride powder and granules as the embedding material, it should at least be possible to prevent the embedding material and the sintered body from sticking together.
However, since it is known that porosity or irregular density distribution impairs the thermoemissive properties of lanthanum hexaboride, attempts have been made to improve the sintering behavior of LaB.sub.6 powders by adding small quantities of nickel or iridium. However, the rate of evaporation under the conditions of use is increased by the nickel boride that is present in the shaped body.
Ceramic mixed materials of boron carbide that contain from 10 to 70% by volume of lanthanum hexaboride and can be manufactured by pressureless sintering of the powder mixtures at 2040.degree. C. have also been used as a cathode material. However, since the emmisivity of these mixed materials is linearly dependent on the LaB.sub.6 content, the high radiation power of pure LaB.sub.6 cannot be fully utilised (see U.S. Pat. No. 4,265,66).
In Inst. of Mat. Science, Acad. of Sciences of the Ukr. SSR, translated from Poroshkovaya Metallurgiya 220 (1981), pp. 56-60, Yu. B. Paderno et al. draw attention to the decomposition of sintered shaped bodies of LaB.sub.6 caused by the use of commercially pure lanthanum hexaboride powders for the manufacture of these shaped bodies. Because of the impurities contained in these powders or formed therefrom during sintering at from 1800.degree. to 1900.degree. C., the sintered bodies contain foreign phases which readily hydrolyse on storage at room temperature. These foreign phases are believed to be carbon-containing lanthanum compounds, such as lanthanum carbides and lanthanum borocarbides, or oxygen-containing compounds, such as lanthanum oxides, boron oxides and borates. Boron oxides and lanthanum oxides have been detected in sintered shaped bodies prepared from commercially available lanthanum hexaboride by x-ray diffraction analysis. Due to the hygroscopic nature of boron oxides and lanthanum oxides, sintered shaped bodies containing these materials break and disintegrate when stored at room temperature.
According to the known processes, it was not possible to manufacture polycrystalline sintered bodies having a high LaB.sub.6 content that have both high density, that is a low porosity, and also a uniform fine-grained microstructure and are free from foreign phases that impair the storage-stability of the sintered bodies at room temperature or increase the rate of evaporation under the conditions of use as a cathode material.
It is therefore interesting from a commercial perspective to provide shaped bodies having a high density and uniform fine-grained microstructure manufactured by a simple pressureless sintering process without having to be embedded in a lanthanum hexaboride powder material, and can use commercially pure lanthanum hexaboride powder as the starting material.